1. Field of the Invention
The present invention relates to a torque sensor and a method of manufacturing the same, and particularly to a torque sensor that is mounted in an electric power steering apparatus of a vehicle or the like, and is used as a detection device to magnetically detect steering torque applied to a shaft through a steering shaft, and a method of manufacturing the torque sensor.
2. Description of the Related Art
In an electric power steering apparatus mounted in a vehicle, generally a torque sensor detects steering torque applied to a shaft through a steering shaft by a steering operation of a driver. In the electric power steering apparatus, a motor is driven to provide assist force (to apply assist steering force) for a steering operation of a driver in response to a detection signal from the torque sensor, so that steering force required by a driver is reduced and a comfortable steering feeling is provided for the driver.
As an example of a torque sensor mounted in such an electric power steering apparatus, there has been known a magnetostrictive torque sensor, which is disclosed, for example, in Japanese Laid-open Patent Application No. 2004-239652 (paragraph [0024˜0034], FIGS. 1, 7˜9).
In the magnetostrictive torque sensor, a magnetostrictive film (for example, a Ni—Fe alloy film) having magnetic anisotropy is attached on the surface of a shaft. When torque is applied to the shaft from outside, the magnetostrictive torque sensor magnetically detects changes of a magnetic characteristic (magnetic permeability) of the magnetostrictive film, which is generated in accordance with a twisting of the shaft, without contacting the magnetostrictive film.
A description will be given on the conventional magnetostrictive torque sensor by taking as an example a magnetostrictive torque sensor mounted in an electric power steering apparatus of a vehicle.
FIG. 9 schematically illustrates the conventional electric power steering apparatus including the torque sensor.
As shown in FIG. 9, an electric power steering apparatus 101 mainly includes a torque sensor 103 that detects steering torque applied from a steering wheel 102, a motor 104 that applies assist steering force for an operation of the steering wheel 102, a reduction gear 105 that increases rotational torque generated by the motor 104, an electrical control unit (ECU) 107 that drives the motor 104 in response to a detection signal from the torque sensor 103 or a vehicle speed sensor 106, a rack shaft 109 and a pinion gear 110 that transmit rotation of the motor 104 to wheels 108, 108, and change a direction of the wheels 108, 108. The reduction gear 105 includes a worm 105A and a worm wheel 105B.
In the electric power steering apparatus 101 configured as above, steering torque applied by a steering operation of a driver is detected by the torque sensor 103, and the motor 104 is driven by the ECU 107 in response to a detection signal (torque signal) from the torque sensor 103, a detection signal (vehicle speed signal) from the vehicle speed sensor 106, or the like. Rotational torque generated by the motor 104 is increased by the reduction gear 105, and then is transmitted to the wheels 108, 108 through the pinion gear 110 and the rack shaft 109.
In the electric power steering apparatus 101, assuming that steering torque applied by a steering operation is TH, rotational torque transmitted to the pinion gear 110 is TP, and a constant that associates with an amount of assist torque generated by the motor 104 is KA, thereby obtaining the equation TH=TP/(1+KA). Therefore, it is possible to reduce steering force required by a driver by using the assist torque generated by the motor 104.
Next, a description will be given on the torque sensor 103 with reference to FIG. 10.
FIG. 10 is a sectional view of a main part of a conventional electric power steering apparatus where a magnetostrictive torque sensor is mounted.
As shown in FIG. 10, the torque sensor 103 is contained in a sensor housing 111 that includes an upper housing 111A and a lower housing 111B. The torque sensor 103 mainly includes a shaft 114 that is connected to a steering shaft 102A (see FIG. 9) of the steering wheel 102 (see FIG. 9) and is rotatably supported in the sensor housing 111 via a shaft bearing 112 or the like, coil units 121, 122 having coils (detection units) 115, 116 that are disposed separately in the axial direction on the inner periphery side of the upper housing 111A, and magnetostrictive films 117, 118 that are formed on the outer periphery side (outer surface) of the shaft 114 so as to face the coils 115, 116 and that have magnetic anisotropy in opposite directions to each other.
When steering torque is applied to the shaft 114 from the steering wheel 102 (see FIG. 9), each of the coils 115, 116 electrically detects changes of a magnetic characteristic of the magnetostrictive films 117, 118 in accordance with a twisting of the shaft 114, and based on the detection result, a direction and an amount of steering torque applied to the shaft 114 are detected.
The upper housing 111A is made of a light alloy, such as an aluminum alloy or a magnesium alloy. Yokes 119, 120 of the coil units 121, 122 are made of a magnetic steel sheet or the like, which has a high magnetic characteristic and a soft magnetic property. The shaft (steering shaft) 114 is made of an iron alloy such as a SC or SCM material, and on the outer periphery side of the shaft 114 are formed the magnetostrictive films 117, 118 of Fe—Ni or Fe—Cr by plating, vapor deposition, or the like.
In the torque sensor 103 disclosed in the above publication, however, the upper housing 111A of the sensor housing 111, the yokes 119, 120, and the shaft 114, which are shown in FIG. 10, are made of different metallic materials, thereby having a different coefficient of linear expansion.
Compared to an iron alloy of the magnetostrictive films 117, 118 and the shaft 114 and a magnetic steel sheet of the yokes 119, 120, an aluminum alloy of the sensor housing 111 has approximately twice the coefficient of linear expansion as an iron alloy and a magnetic steel sheet.
The electric power steering apparatus 101 of a vehicle is used in an environment at a temperature of −40° C. to 80° C., and can be used at a more severe temperature.
At a high temperature, a gap grows between the sensor housing 111 and the yokes 119, 120 due to an expansion of the sensor housing 111 relative to the yokes 119, 120.
In the torque sensor 103, the amount of pressure between the yokes 119, 120 and the sensor housing 111 decreases due to the gap, thereby causing backlash between the yokes 119, 120 and the sensor housing 111. As a result, in the torque sensor 103, a gap “a” between the yokes 119, 120 and the magnetostrictive films 117, 118 changes, and the change of the gap “a” varies a magnetic path, thereby changing an output from the torque sensor 103. For this reason, detection accuracy of the torque sensor 103 is deteriorated. Further, an output from the torque sensor 103 varies because the shaft 114 is offset in the axial direction, that is, the coils 115, 116 slides relative to the position of the magnetostrictive films 117, 118 in the axial direction. For this reason, in the torque sensor 103, a holding portion of the coil units 115, 116 must be processed with high dimensional accuracy.
In order to reduce the backlash between the yokes 119, 120 and the sensor housing 111, the torque sensor 103 will have increase in the number of components and man-hours for assembling, and the complexity of the configuration.